Yeah, conventional x-ray screenings aren't good at detecting anything softer than bone. Given how "stretchy" yeerks are, if they have any bones or other hard bits, they're really small. A yeerk would be really faint on an x-ray, and probably indistinguishable from normal brain tissue. They might show up on a CT scan, but they would still be hard to identify. But I agree that they would probably be visible on an MRI.